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With the development of the Chain Home Radar system during the late 1930s it became clear that the use of centimetric wavelengths would lead to improved radar performance. Soon after the outbreak of War in 1939, a research team at Birmingham University was asked to develop a generator of radio waves operating at a wavelength of about 10 centimetres.

Two members of the team, John Randall and Harry Boot began work with a magnetron in which they used cavity resonant circuits built into the magnetron itself. On 21 February 1940 the cavity magnetron succeeded. In fact, the power generated was so great that the experimenters could hardly believe that it was microwave power.

This experimental magnetron became a working reality through the work of Dr E.C.S. Megaw and his associates at the GEC Research Laboratories, Wembley. Founded by C. C. Paterson the laboratories at Wembley had been opened in 1922 to pursue research independently of product manufacturing. On the outbreak of war GEC put the Laboratories at the disposal of the Government with no consideration of commercial gain or industrial rivalry to hinder the war effort.

Work was undertaken because it seemed that the staff had the skills required to carry it out, not because GEC might gain commercially from it. Indeed several times work started at Wembley was taken through to production by other organisations.

The cavity magnetron was one example of this. Staff at the Wembley laboratories first heard about it in April 1940. Randall and Boot's magnetron was a laboratory device yielded only low power when operated continuously, and was located in the magnetic field of a laboratory electro-magnet producing a steady field. The Wembley staff introduced developments which produced a high-power, pulsed, sealed valve, using a permanent magnet with a new type of cathode, suitable for operational use.

The prototype of this design was tested on 29 June 1940. Others were quickly produced in the laboratories, as the wartime need for the magnetron was so urgent that there was no time to transfer a design to a factory before early demands had been met. One of the early GEC magnetrons was flown to the United States and demonstrated there by members of the Tizard mission.

After the war it was said in the United States of the cavity magnetron "When the Tizard Mission brought one to America in 1940 they carried the most valuable cargo ever brought to our shores".